Directly modulating the analog intensity of a light-emitting diode (LED) or semiconductor laser with an electrical signal is considered among the simplest methods known in the art for transmitting analog signals, such as sound and video signals, on optical fibers. Although such amplitude modulation techniques have the advantage of significantly smaller bandwidth requirements than baseband digital modulation or frequency modulation, amplitude modulation may suffer from noise and nonlinearity introduced by the optical source.
Distortion inherent in certain analog transmitters prevents a linear electrical modulation signal from being converted linearly to an optical signal, and instead causes the signal to become distorted. These effects are particularly detrimental to multi-channel video transmission which requires excellent linearity to prevent channels from interfering with one another. A highly linearized analog optical system has wide application in CATV, interactive TV, and video telephone transmission, for example.
Linearization of optical and other nonlinear transmitters has been studied for some time, but proposed solutions suffer from practical disadvantages or cost penalties that limit usefulness to high value devices. Feedforward techniques, for example, require complex system components such as optical power combiners and multiple optical sources.
One method employed in the past to reduce distortion inherent in nonlinear devices has been predistortion. In this technique, a modulation signal is combined with a signal equal in magnitude to the distortion inherent in the nonlinear device but opposite in sign. When the nonlinear device is modulated by the combined signal, the device's inherent distortion is canceled by the distortion signal generated by the predistortion, and only the linear part of the source signal is transmitted. The intermodulation products in the predistortion signal are at frequencies that are additive and subtractive combinations of integer multiples of the input frequencies. In the distribution of AM signals for cable television, for example, there are often as many as 80 frequencies on a particular band and plenty of opportunities for second order and third order intermodulation products of those frequencies.
Current predistortion techniques generally divide an input signal into two or more electrical paths and generate predistortion on one or more of the paths resembling the distortion inherent in the nonlinear transmitting device. The generated predistortion is the inverse of the nonlinear device's inherent distortion and serves to cancel the effect of the device's inherent distortion when recombined with the input signal before application to the nonlinear device.
Advanced multi-path predistortion circuits are flexible and highly effective for linearizing output of a wide range of nonlinear devices. One such multi-path predistortion circuit is disclosed in U.S. Pat. No. 4,992,754, issued to Blauvelt et al. The circuit is capable of generating frequency specific distortion products for compensating frequency-dependent nonlinearities, and is useful for applications requiring an exceptionally high degree of linearity, such as, for example, CATV applications.
Although multi-path predistortion circuits can be used in a broad variety of applications, the design of these circuits is relatively complex. This complexity manifests itself in circuits that are often too expensive for applications needing only a modest degree of linearization. One skilled in the art would appreciate a low-cost circuit of relatively simple design for limited application, particularly if such a circuit were fabricated from existing low-cost components commonly used in signal transmission applications.
Those skilled in the art would also appreciate a circuit that could produce frequency dependent third-order distortion. Simple third-order distortion, such as that produced by an ideal diode, has the property that the distortion is real and independent of frequency. Many non-linear transmitters or amplifiers, however, contain reactive elements such as inductances, capacitances or delays, which cause the device to produce distortion depending on the input and output frequencies and the distortion frequencies. Nazarathy, U.S. Pat. No. 5,161,044, discloses a circuit in FIG. 15 of that patent which produces essentially real, frequency-independent predistortion. The capacitors and inductors in Nazarathy are added for biasing purposes and to block the DC and AC currents. However, the circuit disclosed by Nazarathy may not have the right phase or frequency dependence for each set of input frequencies, to be substantially the same in magnitude and opposite in sign to the distortion produced by the non-linear device.
The present invention accordingly is addressed to a low-cost predistortion circuit reducing second and higher order distortion products produced by a nonlinear device and to a circuit for generating frequency dependent third-order distortion.